Certain users of hearing aids have a mild to moderate loss of normal sensitivity for low intensity sounds, but retain excellent speech discrimination capabilities and good sound quality judgment capabilities for sounds which are intense enough to be audible to them. This phenomena, commonly referred to as loudness recruitment was, for example, discussed by Steinberg and Gardner in "The Dependence of Hearing Impairment on Sound Intensity", Journal of the Acoustical Society of America, Vol. 9, Pages 11-23 (1937). Accordingly, those hearing aid users who have a hearing impairment characterized by a mild to moderate loss of sensitivity for low intensity sounds require no amplification for high intensity sounds; and, amplification of high intensity sounds will generally cause them discomfort. This problem is well understood in the art, and a variety of solutions have heretofore been employed. However, most of the prior art solutions introduce deterioration in the quality of the delivered sound, so that traditional hearing aids have often been characterized as "low fidelity instruments".
One prior art approach has been to limit the saturation sound pressure output of the hearing aid by means fo a simple peak clipping, which produces large amounts of harmonic and intermodulation distortion. More recently, relatively low distortion compression limiting has been used to limit the saturation sound pressure output of the hearing aid without the introduction of large amounts of harmonic or intermodulation distortion. With either type of output limiting, the saturation sound pressure level out of the hearing aid can be set to a level somewhat below the discomfort level of the user. A typical input-output curve for an output limiting hearing aid is shown in FIG. 4. One disadvantage to the output-limiting approach is that a large percentage of the incoming sounds will be amplified to a point which is near the user's discomfort level.
Another prior art approach has been to use an automatic volume control (AVC) circuit which acts to produce a nearly constant output level for a wide range of input sound pressure levels. In this case, the nearly constant output level can be set to the user's most comfortable listening level, so that the majority of useful sounds are presented at the most comfortable listening level for the user. A typical input-output curve for an automatic volume control hearing aid is shown in FIG. 4. A problem with this latter approach is that the normal dynamic range of the desired sounds is unduly reduced at the output, so that quiet input sounds and background noises are amplified until they appear nearly as loud to the user as loud input sounds. This excessive reduction of the dynamic range lends an unnatural quality to the sound.
A more sophisticated solution provided by the prior art has been the use of wide dynamic range compression amplification wherein the overall gain of the hearing aid is changed smoothly and continuously from a maximum value for low level inputs to a minimum value for high level inputs. Compression hearing aid amplifiers are commercially available which have a nearly constant compression ratio of 2:1 over an input dynamic range of 60 dB or more. With a 2:1 compression ratio, for example, such 10 dB increase in input level causes only a 5 dB increase in output level. An early version of such a compression amplifier suitable for hearing aids was described in U.S. Pat. No. 3,229,049, issued to Hyman Goldberg on Jan. 11, 1966. A typical input-output curve for prior art logarithmic compression hearing aids is shown in FIG. 4. A drawback to such prior art devices, however, is that they provide a compression function characterized by an essentially constant compression ratio for all sound levels up to those sufficiently intense to cause overload of the input circuit, which commonly occurs at microphone input levels corresponding to a hearing level of approximately 80 dB (input sound pressure levels of 90 to 95 dB). For sounds whose sound pressure level exceeds 90 to 95 dB, the overload of the input circuitry causes a rapid increase in amplifier distortion. Since sounds whose peak levels exceed 90 to 95 dB are commonly encountered at concerts, social gatherings, etc., the high distortion levels of such prior art amplifiers renders them unsuitable for use in a high fidelity hearing aid.
Also high quality compression circuits or systems for use in broadcast and recording studio applications are known in the art, see, for example, U.S. Pat. Nos. 3,681,618 and 3,714,462, both to David Blackmer. In these latter systems, the compression ratio and the lower and upper threshold of compression can be readily adjusted by the user to optimize the overall sound quality with different program materials. Such prior art systems have generally required power supply voltages and currents which would be impractical in a head-worn hearing aid; more specifically, power supply requirements of plus and minus 12 volts at a total current drain of 10 mA or more is not uncommon for such prior art systems.
Accordingly, it is a principal object of the present invention to provide a high fidelity hearing aid.
It is another object of the present invention to provide a very low noise, low distortion, variable gain preamplifier capable of handling the unattenuated output of a subminiature microphone without distortion for input sound pressure levels of 110 dB SPL or greater.
It is another object of the present invention to provide a low distortion, low noise, variable compression ratio circuit which requires a supply voltage of 1.5 volts, or less, and a supply current of approximately 100 microamperes, or less.
It is a further object of the present invention to provide a preamplifier whose gain in dB is linearly related to the logarithm of the output signal voltage over an input range of at least 40 dB.
It is yet another object of the present invention to provide a preamplifier circuit with the compression ratio and the maximum gain of the preamplifier adjustable by the user in a simple manner without affecting the ability of the circuit to provide low distortion operation.
A further object of the present invention is to provide a circuit for obtaining wide dynamic range logarithmic compression acting to increase circuit gain for low level signals, in combination with fast-acting, low distortion output limiting compression acting to attenuate the microphone output when output SPL's exceed some predetermined level, and wherein the two separate compression actions do not interact in an undesirable manner.
These and other such objects as may hereinafter appear are attained in the embodiments of the invention shown in the accompanying drawings wherein: